Self-Diffusion and Correlation Effects in Ordered AuZn Alloys

Abstract

Simultaneous self-diffusion measurements of Zn and Au in near-equiatomic highly ordered AuZn ($B2$ structure) single-crystal specimens having 49.0, 50.0, 51.0 at.% Zn nominal composition were made over a temperature range of 428-650 \ifmmode^\circ\else\textdegree\fi{}C using ${\mathrm{Zn}}^{65}$ and ${\mathrm{Au}}^{195}$ radioactive tracers and the serial lathe sectioning technique. Employing a modified energy-discrimination method for nuclear counting, it was possible to measure the ratio of diffusivities $\frac{{D}_{\mathrm{Zn}}}{{D}_{\mathrm{Au}}}$-in addition to their individual values-following simultaneous diffusion of the two radioactive tracers in the same specimen. The diffusion parameters $Q$ and ${D}_{0}$ and the entropy of activation factor were computed in the composition range studied and compared with the corresponding quantities in the disordered Zn-base monovalent alloys ($A2$ structure). The magnitude of the ratio of the correlation factors in ordered and disordered phases was found to be consistent with the correlated six-atom-vacancy-jump diffusion mechanism in the highly ordered alloys. The self-diffusion data for Au and Zn in ${\ensuremath{\beta}}^{\ensuremath{'}}\ensuremath{-}\mathrm{AuZn}$ alloys also provides strong evidence for the presence of a nonequilibrium vacancy-defect structure on the Zn-rich side of stoichiometry.